PROBABILISTIC-BASED HURRICANE RISK ASSESSMENT AND ...

degradation due to the ageing of utility poles in Norway can be represented with anexponential decay function.To model the degradation of the strength of distribution poles, the resistance within thelimit state function (e.g. Eq. 1.1) of the pole must be modified to represent how strengthdeteriorates over time. The modified resistance (Eq. 1.13) is estimated for a specific ageof pole, and used in Eq. (3) to estimate the probability of failure after a specific timeframe.R(t) = a(t) ∙ ME ∙ R n (1.13)where R(t) is the resistance after time t, a(t) is the degradation function, ME is the modelerror to take into account that the design values are not equal to the actual values, and R nis the design strength of the pole.The degradation function (a(t)) is estimated with Eq. (1.14).a(t) = π (D − 32 d(t))3 (1.14)where D is the initial diameter of the pole and d(t) is the decay depth after time t.Leicester et al. (2003) developed an engineering model that estimates the depth of decayin timber poles after a period of time, based on a comprehensive study of timberdegradation. Wang et al. (2008b) took the model further and developed a model thatestimates the strength of timber poles after a specific time period based on the decaydepth. These models were developed based on a survey of timber poles in Australia, butwill be used herein as a starting point to estimate the degradation of timber pole strengthin the U.S.Timber poles are typically composed of either solely heartwood or a combination ofsapwood and heartwood. For simplicity, it is assumed herein that the timber distributionpoles are composed of only untreated heartwood. According to the Australian Standardon Timber Durability (AS 5604-2005), the Durability Class of timber is assigneddepending on the expected service life (Morrell 2005), which means that it is a DurabilityClass 2 according to AS 5604 (2005). The hardness of Southern pine is classified as soft(AS 5604-2005); these characteristics will be assumed to apply to Southern pine timber26